172 THE BELL SYSTEM TECHNICAL JOURNAL, JANUARY 1957 



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10 20 30 40 50 60 70 80 90 100 110 120 



PLATE VOLTAGE 



Fig. 5 — Typical plate voltage-plate current characteristics for a type 175HQ 

 tube. 



Immediate concern for the lowering of insulation resistance across mica 

 spacers prompted an investigation. The source was traced to the use of 

 plates made from a grade of nickel from which magnesium as a contam- 

 inant was evaporating. A change was made to molybdenum which has 

 been used successfully since that experience. 



The effect on the thermionic life of operating at different cathode cur- 

 rent densities was of interest. Life tests were started in which the cathode 

 current drain in one group of tubes was approximately 7.5 milliamperes 

 (2.8 ma/cm^) and in another group the average cathode current was 0.6 

 milliamperes (0.2 ma/cm^). The results presented in Fig. 6 after 120,000 

 hours, or approximately 14 years, show that at the cathode temperature 

 of approximately 710°C* selected for the test, there is practically no 

 current density effect in this 12 to 1 current range. The circles indicate 

 average values, while the dots and crosses at each test point show the 

 positions of the extreme tubes of the group. 



Similar life tests set up to show the effect of operating at plate and 

 screen voltages of 60 volts as compared to 40 volts indicate no essential 

 differences in performance after 8 years of operation. 



* All cathode temperatures referred to in this paper are "true" temperatures, 

 not imcorrected pj^rometer temperatures. 



